Use a sharp paring knife to carefully slice a horizontal pocket into each chicken breast, avoiding cutting all the way through. Gently flatten the chicken if needed with a meat mallet, then season inside and out with salt and pepper.
In a bowl, combine the cooked spicy chorizo, shredded mozzarella, chopped herbs, and minced garlic. Mix until well combined and the ingredients are evenly distributed.
Using a small spoon, stuff each chicken pocket with the chorizo mixture, pressing gently to seal the edges. Secure with toothpicks if needed to prevent filling from leaking out during cooking.
Heat olive oil in an oven-proof skillet over medium-high heat until shimmering and just starting to smoke. Sear each stuffed chicken breast for about 3-4 minutes per side until golden brown and crispy, listening for a satisfying sizzle and watching for a rich, golden crust.
Transfer the skillet to a preheated oven at 190°C (375°F). Bake for 20-25 minutes, or until the chicken reaches an internal temperature of 75°C (165°F) and the filling is bubbling hot and melted.
Remove the skillet from the oven and let the chicken rest for 5 minutes. Carefully remove toothpicks and slice the chicken if desired, revealing the gooey, spicy filling inside. Serve hot, spooning pan juices over the top for extra flavor.
